1. Field of the Invention
The present invention relates to a liquid crystal display device for use in a display unit of electronic appliances.
2. Description of the Related Art
For a liquid crystal display device which can attain a wide viewing angle, a liquid crystal display device in MVA (Multi-domain Vertical Alignment) mode is known. The liquid crystal display device in the MVA mode has liquid crystals of negative dielectric anisotropy which are sealed between a pair of substrates, a vertical alignment film which aligns liquid crystal molecules almost vertically with respect to the substrate surface, and an alignment regulating structure which regulates the alignment orientation of liquid crystal molecules. For the alignment regulating structure, a linear protrusion formed of a dielectric and an open part (main slit) of an electrode are used. When voltage is applied, the liquid crystal molecules are tilted in the direction vertical to the direction in which the alignment regulating structure is extended. The alignment regulating structure is used to provide a plurality of areas in a single pixel, the area being different in the alignment orientation of the liquid crystal molecules, and thus a wide viewing angle can be obtained. However, in this liquid crystal display device, since the protrusion and the main slit are formed in the pixel area, the aperture ratio is lower than ones in the TN (Twisted Nematic) mode etc., and thus the light transmittance is reduced.
FIG. 34 shows the pixel structure of a liquid crystal display device in the MVA mode in which the aperture ratio is improved. FIG. 35 shows the sectional structure of the liquid crystal display device sectioned at line A-A shown in FIG. 34. As shown in FIGS. 34 and 35, the liquid crystal display device has a pair of substrates 102 and 104, and liquid crystals 106 which are sealed between the substrates 102 and 104. On the substrate 102, a gate bus line 112 and a drain bus line 114 are formed as they intersect with each other through an insulating film. A pixel area is defined by the gate bus line 112 and the drain bus line 114. Near the position at which the gate bus line 112 and the drain bus line 114 intersect with each other, a TFT 120 is formed. In each of the pixel areas, a pixel electrode 116 is formed. The pixel electrode 116 is formed with a micro slit 116d which is cut from a rim. The alignment orientation of the liquid crystals molecules 108 is controlled by an oblique electric field at the end of the pixel electrode 116. In this liquid crystal display device, a high aperture ratio and light transmittance can be obtained because the linear protrusion and the main slit are not formed in the pixel area. However, since the micro slit 116d has weaker alignment control than that of the linear protrusion and the main slit, the liquid crystal display device has a long response time of the liquid crystals, and the alignment is easily disturbed by a finger press or so.
Then, a polymer sustained alignment (PSA) technique is introduced in which polymerizable monomers are mixed in liquid crystals to polymerize the monomers in the state in which voltage is applied to the liquid crystals and thus the orientation in which the liquid crystals are tilted is stored (for example, see Patent Document 1). In the liquid crystal display device using the PSA technique, a polymerized film which stores the alignment orientation of the liquid crystals is formed on the interface of an alignment film. Thus, strong alignment control is obtained, and it can be ensured that liquid crystals molecules 108 are tilted in the direction in parallel with the micro slit 116d. 
However, in a liquid crystal display device in the VA mode in which the birefringence property of the liquid crystals vertically aligned is used for switching light, the phase difference caused by birefringence in the oblique direction is greatly shifted from that in the front direction, and thus display can have a white patch when a screen is viewed from the oblique direction. This is a phenomenon called washout in which the gray level brightness characteristic, that is, the γ characteristic is shifted from a set value in all the gray scale levels more or less.
For a scheme to improve washout, there is a technique in which a single pixel is split into a plurality of areas to vary the voltage applied to liquid crystals in a single pixel. This is a technique in which the alignment orientation of liquid crystals is varied in the azimuth angle direction as well as in the polar angle direction to reduce the shift of the phase difference between the oblique direction and the front direction. More specifically, the alignment orientation of liquid crystals in a single pixel is split in the polar angle direction as well as in the azimuth angle direction to average the variation in the phase difference in the polar angle direction as well, and thus a white patch can be reduced.
FIG. 36 shows the pixel structure of a liquid crystal display device which implements the technique above. As shown in FIG. 36, a pixel electrode in each of pixel areas has a direct coupling part 116a which is directly connected to a source electrode of a TFT 120, a capacitive coupling part 116b which is indirectly connected to the source electrode through capacitance formed between it and a control capacitance electrode 125, and a space 117 which isolates them. The direct coupling part 116a and the capacitive coupling part 116b each have a plurality of line electrodes (width l) which is extended in a predetermined direction, and a micro slit (width s) which is between the adjacent line electrodes. Near the space 117, the micro, slits of the direct coupling part 116a and the micro slit of the capacitive coupling part 116b are extended almost in parallel with each other. In the configuration shown in FIG. 36, the voltage applied to the liquid crystals are varied between the direct coupling part 116a and the capacitive coupling part 116b to obtain an effect to reduce a white patch.
However, this mode has a problem that a potential difference is generated between the direct coupling part 116a and the capacitive coupling part 116b and that potential difference causes a shift of the alignment orientation of liquid crystals in the space 117 from the orientation defined by the micro slit of the pixel electrode. FIG. 37A shows the pixel electrode structure near the space 117. FIG. 37B shows the simulation result of the display state of a pixel. As shown in FIG. 37A, since a smaller voltage is applied to the capacitive coupling part 116b when it is driven than that to the direct coupling part 116a, the electrode end of the direct coupling part 116a works just like a main slit, and the tilt orientation of the liquid crystal molecules is temporarily vertical to the electrode end of the direct coupling part 116a. On this account, the alignment at the direct coupling part 116a and the capacitive coupling part 116b near the space 117 is greatly disturbed. This phenomenon is called azimuth angle (φ) fluctuations. When the φ fluctuations occur, the birefringence properties of the liquid crystals are locally reduced to generate a dark line as shown in FIG. 37B. Thus, the light transmittance of a pixel is decreased. In addition, the shift of the alignment orientation of liquid crystals also affects the viewing angle characteristic to reduce the effect that improves the white patch described above as well. In order to reduce the influence on the viewing angle characteristic, it is necessary to shield light in the space 117 between the direct coupling part 116a and the capacitive coupling part 116b by a black matrix (BM). On this account, a problem arises that the light transmittance of a pixel is further decreased.
Patent Document 1: JP-A-2003-149647
Patent Document 2: JP-A-2003-177408